The exchange of information through wireless communications systems continues to grow with the increase of pagers, cellular telephones, wireless local area networks (WLANs) and other wireless communication devices and networks. A majority of the wireless communications systems include a transmitter and a receiver using a superheterodyne architecture employing a low intermediate frequency (IF). In a low IF superheterodyne receiver, a received radio frequency (RF) signal is downconverted to a low IF by two mixers driven by quadrature phases of a local oscillator. Instead of using a low IF, some wireless communications systems may use a zero IF or direct conversion architecture which also employs quadrature mixing. For more information regarding zero IF receivers, see Asad A. Abidi, Direct-conversion Radio Transceivers for Digital Communications, IEEE Journal of Solid-State Circuits, Vol 30, No. 12, December 1995 at 1399, which is incorporated herein by reference.
Nevertheless, both low IF and zero IF receivers employ quadrature mixing and should address a common problem of matching or balancing the amplitudes and phases of an in-phase and quadrature phase branch, or channel, of the received RF signal. The mismatch between the two channels may be the result of bipolar transistors, MOS transistors and finite tolerances of capacitor and resistor values used to implement the analog components of the receiver. While balanced channels in a quadrature mixing receiver correspond to a pure frequency translation, any mismatches introduce a frequency translation that results in a mixture of an image and a desired signal when the in-phase and quadrature phase channels of the received RF signal are not exactly 90 degrees out of phase, and the image is seen as interference.
In Mikko Valkama, et al., Advanced Methods for I/Q Imbalance Compensation in Communication Receivers, IEEE Transactions on Signal Processing, Volume 49, NO. 10, Oct. 2001, at 2335, which is incorporated herein by reference, several methods have been proposed for imbalance compensation for in-phase and quadrature phase channels using baseband digital signal processing. One suggested method involves a structure based on a traditional adaptive interference canceller. Improved image rejection may also be obtained by using advanced blind source separation techniques, which do not require known training signals.
The proposed image rejection methods, however, concentrate on an application wherein the receiver imbalance properties are solely frequency independent. Nevertheless, image rejection in quadrature mixing receivers is also created by frequency dependent sources. Addressing both frequency dependent and independent causes of image rejection in quadrature receivers increases the complexity of a solution. The suggested methods of imbalance compensation either require complex and expensive structures or do not sufficiently reduce the imbalance between the in-phase and the quadrature phase channels of quadrature mixing receivers. Furthermore, the suggested methods of imbalance compensation do not sufficiently address the degradation of the signal-to-noise ratio of the received RF signal for higher data rates of, for instance, 54 Mbits/s and up.
Accordingly, what is needed in the art is an improved system and method for reducing a degradation in the performance of quadrature mixing receivers including a reduction in the image rejection associated therewith.